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The Dynamics of fluid flow and associated chemical fluxes at active continental margins


Active fluid flow plays an important role in the geochemical, thermal, and physical evolution of the Earth's crust. This dissertation investigates the active fluid flow and associated chemical fluxes at two dynamic continental margins: The Costa Rica subduction zone and the northern Gulf of Mexico hydrocarbon province, using novel seafloor instrumentation for continuous monitoring of fluid flow rates and chemistry. Traditional pore fluid sampling methods and flow rate models only provide a steady-state view of these types of hydrogeochemical systems. The data presented in this thesis, however, show that these systems are dynamic with short period transients in both flow rates and chemistry having a wide range of implications. Osmotically pumped fluid samplers and a new borehole fluid flow meter were deployed in CORKed borehole observatories installed in two active hydrogeochemical systems at the Costa Rica subduction zone. The data collected by this effort provide critical information on the average and transient hydrogeochemical state of the upper igneous basement and subduction thrust. The results suggest that the upper igneous basement provides an efficient pathway for fluids expelled from the forearc that should be included in models of subduction zone hydrogeology and deformation. Two transients in flow rates along the décollement are the result of aseismic slip dislocations that ruptured through the seismogenic zone. These observations provide insight into the dynamics of episodic strain and the associated fluid flow along the décollement, and may provide an analog for the strain and flow response prior to and during a large subduction zone earthquake. Recent studies have shown that when the subduction flux of incompatible elements, such as Ba, in the incoming sediment section is high, the arc volcanoes erupt lavas enriched in these elements. Mass balance calculations based on the sedimentary input flux of Ba at reference sections drilled seaward of the trench and the enrichment of Ba in volcanic arcs, suggest 20-30% of the subducted sediment Ba budget is recycled to the arc. A detailed analysis of pore fluid and sediment Ba concentrations from Ocean Drilling Program (ODP) cores collected at the Costa Rica subduction zone, shows that barite dissolution in the forearc can have a profound impact on the amount of Ba recycled to the arc volcanoes and mantle. At the Costa Rica margin, approximately 60% of the incoming sedimentary Ba is lost in the shallow subduction zone, greatly reducing previous estimates of the Ba input flux to the depths of magma generation based solely on the reference section seaward of the trench. These results suggest that current estimates of the global sediment Ba contribution to volcanic arcs should be reevaluated, and may need to be revised upwards. Four newly designed fluid flow meters/chemical samplers, called the MOSQUITO, were deployed for 430 days at a seafloor gas hydrate outcrop in the northern Gulf of Mexico to determine how dynamic subsurface fluid flow influences gas hydrate stability and to quantify the associated methane fluxes into the ocean and atmosphere. The results show that gas hydrate continuously formed within the sediments despite a highly transient fluid flow field and variable bottom water temperatures. A model is proposed where gas hydrate formation occurred as a result of long-term emanation of CH₄ at focused gas vents followed by a more diffuse intergranular methane flux. From the long-term flow rate record, the estimated CH₄ flux across the seafloor to the water column from focused vents at the Bush Hill seep is ̃5.10⁶ mol/yr. The surface water CH₄ concentrations above the perennial hydrocarbon plumes emitted at these high-flux gas seeps are up to 2,000 times supersaturated.The estimated diffusive CHv(4) fluxes to the atmosphere from individual plumes are 3-4 orders of magnitude greater than previously reported from the deepwater marine environment. Extrapolation over the Gulf of Mexico continental shelf and slope indicates that these perennial hydrocarbon plumes emit at least 0.1 to 2.3 Tg CHv(4) yr, and suggests submarine hydrocarbon seeps in the Gulf of Mexico and in other oil-rich regions are globally significant sources of (4)C-dead ("fossil") methane to the atmosphere

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